For a surface of a display panel, such as a cathode ray tube (CRT) display panel, liquid crystal display (LCD) panel, plasma display panel (PDP) and electro-luminescence (EL) display panel, various functions, including a scratch prevention function and a contamination prevention function, are required. Liquid crystal alignment defects may be generated when, for example, the liquid crystal display panel is subjected to an external impacts. Therefore in order to prevent scratches and contamination, and protect from an external impact, in some cases a protective plate is disposed on the surface of the display panel.
If such a protective plate is disposed on the surface of the display panel, light is reflected not only on the surface of the liquid crystal display panel, but also on the surface of the protective plate, and a display may not be viewed well in a bright indoor location or an outdoor location. In particular, in the case when structure of the protective plate becomes complicated by attaching such a function as a touch panel thereto, light reflection quantity tends to increase because of the structure of the protective plate. To prevent this, means for reducing light reflection from the protective plate by disposing a circularly polarizing plate for the protective plate has been disclosed (e.g. see Patent Documents 1 to 3). The principle of reducing reflected light by the circularly polarizing plate is as follows.
FIG. 8 is a cross-sectional view schematically depicting a configuration of a conventional liquid crystal display device on which a circularly polarizing plate is disposed for a protective plate. As FIG. 8 shows, an example of means for reducing reflection of light due to the structure of the protective plate is to use a liquid crystal display device having a touch panel with a circularly polarizing plate 101 on the surface of the liquid crystal panel. In FIG. 8, the touch panel with a circularly polarizing plate 101 is disposed on the observation surface side of the liquid crystal panel 102. The liquid crystal panel 102 has a lower polarizer 133, lower substrate 122, liquid crystal layer 151, upper substrate 121 and upper polarizer 132, which are stacked in this order toward the observation surface. The touch panel with a circularly polarizing plate 101 is disposed on the observation surface side of the liquid crystal panel 102, and includes a touch panel 111, a λ/4 plate 181 and a polarizer 131, which are stacked in this order toward the observation surface. The light that has passed through the stack structure of the polarizer 131 and the λ/4 plate 181 is converted to a circularly polarized light. The stack structure of the polarizer 131 and λ/4 plate 181 is normally called a circularly polarizing plate 191.
The external light (natural light) entered the touch panel with a circularly polarizing plate 101 passes through the polarizer 131 first, and is then converted to linearly polarized light vibrating in one direction, and then passes through the λ/4 plate 181, so that the linearly polarized light is converted to circularly polarized light rotating in one direction (the rotation of the circularly polarized light at this time is assumed to be counterclockwise when viewed from the direction facing the traveling direction). Some of the components, out of the converted circularly polarized light, are reflected on the surface of the touch panel 111 and the surface of the liquid crystal panel 102, and travel backward toward the outside. The circularly polarized light reflected on the surface of the touch panel 111 and the surface of the liquid crystal panel 102 travels in a reverse direction while maintaining the same rotating direction, so it becomes clockwise circularly polarized light when viewed from a direction facing the travel direction. The clockwise circularly polarized light passes through the λ/4 plate again, and is converted to the linearly polarized light again. The vibration direction of the converted linearly polarized light changes to a direction orthogonal to the vibration direction of the incident linearly polarized light. Since the polarizer allows transmitting only light vibrating in one predetermined direction (in this case the vibrating direction of incident linearly polarized light), the light reflected on the surface of the liquid crystal panel 102 in the touch panel 111 is blocked by the polarizer 131, is not emitted to the outside, and is not recognized as reflected light.
In this way, the configuration of the circularly polarizing plate 191 permits the reduction of the reflected light, but the above-mentioned configuration alone is insufficient for the light that has passed through the liquid crystal panel 102. This is because the light that has passed through the liquid crystal panel 102 passes through the λ/4 plate 181 only once, after passing through the liquid crystal panel 102, and by passing through the polarizer 131 after passing through the λ/4 plate 181, transmitted light is greatly reduced and transmittance, which is critical for a display, drops. Therefore according to a conventional liquid crystal display device, a λ/4 plate 182 having an optical axis in a direction orthogonal to the optical axis of the λ/4 plate 181 of the touch panel with a circularly polarizing plate 101, is disposed between the liquid crystal panel 102 and the touch panel 101 with a circularly polarizing plate in the liquid crystal display device, whereby the phase difference of transmitted light is adjusted, and light that has passed through the liquid crystal panel 102 can be emitted to the outside, without dropping the transmittance due to the polarizer 131 of the touch panel with a circularly polarizing plate 101. This principle is also used for an organic EL display panel (e.g. see Patent Document 4).    [Patent Document 1] Japanese Kokai Publication No. H10-48625    [Patent Document 2] Japanese Kokai Publication No. 2002-72214    [Patent Document 3] Japanese Kokai Publication No. 2002-148592    [Patent Document 4] Japanese Kokai Publication No. 2002-311239
However in order to add a function of the circularly polarizing plate to the protective plate, a new λ/4 plate must be fabricated, whereby a certain degree of antireflection effect may be obtained, but adding the λ/4 plate may cause such problems as deterioration of the field of view from the diagonal direction with respect to the display surface of the display panel, and the generation of a certain tint on the display.